Elevated Bridge B/C

[jander14indoor]

Everything you guys mention is true, except one cost of balsa stripper. More on that in a moment.

Yes, density varies greatly across a sheet. The most consistent I ever saw varied about 2 to 1 in density, rare. More common is 4 to 1, I've seen as high as 10 to one. I typically buy sheets near my target density, strip it, weigh each piece, and use accordingly. Some gets discarded, but much just gets used in appropriate spots as needed. Heck, the key thing I emphasize when coaching WS is weight, and show folks how fanatical I am by showing them bundles of hundreds of 1/16 by 3/32 sticks, each with its weight carefully written on one end. I eventually use most of what I strip.

Now, besides cost, here's the real reason I start with sheets. You can see the grain MUCH better than with sticks. This lets you select for straight, even grain. Also lets you avoid hard to see defects like wind shakes. Both of these are important for more consistent physical properties. With experience you can select much more consistent balsa if wanted. This is also the way to find the lightest balsa. Start with a fairly light sheet with very distinct grain. The lighter colored sections will be lower density, as low as 3 lb per foot (though that's rare, but 4 lb per foot is common).

Now, cost of strippers. Yes, there are expensive strippers, I have one in the $150 price range I use for my indoor planes. It lets me strip wood to the nearest .001 inches. That's OVERKILL for all SO events. The stripper I use the most cost me $5 years ago, I think its up to $7 or so now. Its called the Master Airscrew Balsa Stripper. Also capable of stripping to .001 but you have to get there by trial and error. Certainly consistent from strip to strip if you are careful, just harder to adjust than my expensive one.

Now, one of my fellow Miching SO board members here in MI coaches Bridges and swears by sticks. I coach WS mostly, and swear by sheets. Both can succeed, just have to focus in different areas.

Jeff Anderson
Livonia, MI

[nejanimb]

Personally, it seems that sticks make sense, if only because of the time factor. Not having to strip my own wood saves a ton of time, and allows us to simply build and test more structures. My only complain is that, even specializedbalsa doesn't have sticks all the way down to 1/32nd, and I've not seen anyone else that sells anything that small in dimension. 1/16th was fine for this year, but with some design ideas for next year, I might want to try skinner.

[Greg Doe]

I couldn't agree more with Jeff on balsa strippers. I have a Master Airscrew Balsa Stripper that's at least 20 years old, and it's probably only had 3 or 4 blades in it! There are better balsa stripers, but they are not necessary for SO events. Current list price is $7.35. The company that makes it is
Windsor Propeller Co. This is their web site: http://www.masterairscrew.com Most well stocked hobby
shops that sell radio control model airplanes can order one if they don't already have them in stock. 36 inch aluminum rulers are available at hardware stores for a few dollars, and work great for
straighting up the edges of balsa sheets, if it's necessary.
Light weight strip stock is often hard to find at most hobby shops. For what ever reason most balsa suppliers select medium to heavy wood to be cut into strip stock, possibly because there is less waste? This means to get 6 to 10 lb. per cu.ft. wood it has to be ordered. I'll wager that I can strip all the wood needed to build a bridge faster than someone can order it.
Jeff mentioned "wind shakes" but didn't elaborate. Now is a good time to explane this condition
since I haven't seen it mentioned in the Forum this year. I prefer the description of "wind check cracks". Balsa trees contain a lot of moisture (water). Sometimes the trees are subjected to heavy winds which cause the trees to bend to the point that they break. Because they are so moist they don't fracture, but rather collapse like a plastic drinking straw. Later the tree recovers and continues
to grow until it is cut down and dried, but the damage remains. When we try to build something the wood breaks very easily or even falls apart. Most of this defect is sorted out by the wood cutters,
but occasionally defective wood is shipped to retailers. In 50 plus years of building model airplanes
I've only seen "wind shake" damage less than a dozen times.
Balsa is an amazing natural building material, and volumns and have been written about it. Knowing
how to make it's properties work best for the task at hand will separate the winners from the loosers.
Having the ability to strip your own sticks gives you one more tool in your quest for the lightest structure.
Greg Doe
Smyrna, TN

[blue cobra]

Do you have the demensions of the MA? I'm getting it for sure, and I was going to build a "fence" for it. I have a smooth, hard peice of wood for the base, and a strip of wood 1/8 x 3/4 I was going to use for the raised part where you mount the stripper.
On a rather unrelated note, BalsaMan, with the safety tower, would you include the weight of the chain for weight held? Also, what the tower does is make it so no testing equipmet crushes your bridge after it breaks, so you can see where the forces of the weight broke your brigde, correct?

[Dark Sabre]

Random Question:

Short of building (and annually changing) a hopper to test with, is there any way to test a bridge at home that is better than other methods? During the season I can use the school's testing apparatus, but if there is a way to get a very good idea at home without spending much money, I would appreciate if someone told me.

Now to contradict myself, is there a place where I can get a diagram or a description of the loading block (is it a 5x5x2 hunk of anything with a hole in the middle for a threaded rod)? We have quite a bit of materials around our house, and I may actually be able to make a testing device.

You probably figured out whatever you were going to do by now, but I finally brought my loading blocks in from the car to take pics of...

1649|10/a2j_RandomCanon 3109.jpg 1648|10/a2j_RandomCanon 3108.jpg 1647|10/a2j_RandomCanon 3107.jpg

I grabbed a board of oak I had and cut the blocks out of it...rest of the stuff was from Lowes.

[jander14indoor]

Do you have the demensions of the MA? I'm getting it for sure, and I was going to build a "fence" for it. I have a smooth, hard peice of wood for the base, and a strip of wood 1/8 x 3/4 I was going to use for the raised part where you mount the stripper.

Not sure why you'd mount the thing to a board with a fence. It has a built in fence and the handy thing about it is it ISN'T mounted to anything. You start with a straight sheet edge and that does your fixturing. Hold the sheet to a smooth surface, slide the stripper along the edge, cutting halfway through, flip, repeat, a nice square cut strip. As straight as the original edge with practice.

Anyway, its about 4 3/16 inches long by 1 in wide by 7/16 in thick or so. It has an adjustable cross bar to hold the blade and set the width of the strip about 2 in long by 3/4 in wide by 3/8 thick. Quick measure with a ruler.

On a rather unrelated note, BalsaMan, with the safety tower, would you include the weight of the chain for weight held? Also, what the tower does is make it so no testing equipment crushes your bridge after it breaks, so you can see where the forces of the weight broke your brigde, correct?

I'm not BalsaMan, but as an engineer I love the way under appreciated elegance of the testing approach this tower allows. No, the purpose is to apply the load slowly so it breaks slowly. No need for a high speed camera to catch the action. You can spot the first point of failure and STOP. The failure is not obscured by all the secondary failures as they never get a chance to propagate since the load is prevented from falling any further. Fix that spot, continue testing THE SAME BRIDGE to find ALL the weak points. Redesign/rebuild fixing all the things you observed. Fix weak points, lighten over strong ones. And repeat. If that's not clear, look way earlier in this string to find where this is discussed in much depth.

Jeff Anderson
Livonia, MI

Jeff Anderson
Livonia, MI

[blue cobra]

Thanks! I thought a fence might be helpful, but if it's not needed, I really don't don't need one, do I Anyway if I do want one, it's just a few scraps and screws away.
And thanks for you're explanation of the safety tower as well.

[nejanimb]

Balsa Man, I understand if you've still been busy, but did you ever get a chance to grab pictures of the old 3-load bridge? Still would be very curious to see.

[sewforlife]

thanks for remind us. Yes, if you're not too busy, Balsa Man, would you post a picture?

[JimY]

Back on page 96, I shared some efficiencies from our state bridges this season for both B and C. On page 97, baker asked if I’d share a photo of the B division bridge that we took to nationals and won with. By the way, this was my fourth time coaching the winning bridge at nats, all in B division. There was a 4th in there in 2006 too, plus a 5th in Boomilever last season in C division. Best in B division tower was 7th in 2007.

Actually, I don’t have a photo of the bridge. So instead, I’ll give a general description. As mentioned previously, it had 9 structural nodes per truss, which gave 7 interconnected triangles per truss. The node points were at the following locations (in cm from the left bottom node point):
0, 0
8.5, 12.3
13.5, 10.2
16.2, 14.8
18.5, 10.2
20.8, 14.8
23.5, 10.2
28.5, 12.3
37.0, 0

It makes the most sense on graph paper, so try it.

As mentioned previously, it had 98 pieces of wood total out of which 10 were balsa. The 5 load-bearing compression pieces along the top of each truss were bass. These bass pieces were made into L shaped girders with the balsa pieces. The job of the balsa pieces is to stiffen the bass pieces up a bit and also provide gluing surface area for the pieces that connect the trusses together. The balsa pieces carry very little stress, but I will not design a bridge without the perpendicular pieces on the compression beams (but only on the compression beams).

If you calculate the stresses on the interior chords for the node points, you find that only two have high stress and the rest are either low or zero. For the high stress chords, we used two pieces of bass for each chord (four pieces total per truss). The other four chords per truss used one piece of bass each. On the diagonal legs, we also used two stiffeners per leg. These were spaced such that the three resulting sections on the compression leg were each about the same length (I hope this isn’t too confusing).

At the table surface, the tension leg was connected to the compression leg using two gussets plus a spacer to bring the thickness of the thinner piece closer to the thickness of the thicker piece. At the upper corners on the bottom side of the truss, the pieces were lapped together. The high stress chords were lapped from that corner to the other corner.

The trusses were held together with bass pieces. There were 32 of these in total, with 8 directly across the trusses and the other 24 as 12 sets of cross braces. Rounding out the 96 pieces were 4 tiny gussets connecting compression pieces together on the top inside. So here it is again:

20 compression pieces (10 bass in load-bearing position and 10 balsa as perpendicular pieces)
12 gussets and spacers for the legs at the table surface
6 tension members
8 pieces for the high tension chords
8 pieces for the low or zero tension chords
8 stiffeners
4 gussets for the top of the compression pieces
32 pieces between the two trusses

The mass in Indiana was 5.90 grams, and mass in Georgia was 5.96 grams. We didn’t do anything special to keep moisture out.

Yes, we used a jig to hold the trusses while connecting them together. The glue was CA. Bass was purchased over the internet at northeasternscalelumber.com, and balsa at a local hobby store. The bass for the compression legs was actually left over from the 2001 season. Some is cut from sheets, and smaller dimension pieces are purchased pre-cut. While I can’t include the dimensions of the pieces, you can imagine that small rules. I use a spreadsheet to calculate the stresses on all the load-bearing pieces and the approximate mass of the wood. The spreadsheet also calculates the load on each piece in pounds per square inch of either tension or compression. It isn’t terribly sophisticated to an engineer, but it is essentially rocket science for middle school students and most coaches. Anyone that has had first year physics can likely use it without trouble. Sorry, but it isn’t for sale. However, with this tool plus time, you gain a real sense of what the wood is capable of doing without building dozens and dozens of bridges each season.

Total construction time for one is 5-6 hours.

That’s about it.